Route all ManiphestTransaction reads through a single class

Summary:
Ref T2217. I'm going to do the fake-double-writes ("double reads"?) thing where we proxy the storage that worked pretty well for auth. That is:

  - (Some more cleanup diffs next, maybe?)
  - Move all the data to the new storage, and make `ManiphestTransaction` read and write by wrapping `ManiphestTransactionPro`.
  - If nothing breaks, it's a straight shot to nuking ManiphestTransaction callsite by callsite.

I think Maniphest is way easier than Differential, because there are very few query sites and no inline comments.

Test Plan: `grep` to find callsites. Loaded task view, called Conduit.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, chad

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7067